First you can disconnect the battery and try to reset the system. If this doesn't work the computer will need to be scanned for any codes relating to this problem. Here is the following test procedure. Hope this is helpful.
Install a scanner and check for codes in the Heater Ventilation and Air Conditioning (HVAC) system. If the scanner does not have the ability to get into HVAC system, then use a voltmeter to check the feedback from the temperature motor. Back probe the Dark Blue wire at the right temperature motor and make sure the voltage is in the range of 0.5-4.5 volts. If voltage is above or below that specification, then replace the temperature motor and disconnect the battery to reset the system.